Added get agent content files

🎯 Objective

Improve code organization and separation of concerns by extracting new agent-specific file management operations into a dedicated service.

📋 Changes

New Components

• AgentFileManagerService: Dedicated service for new agent-specific file operations
• AgentFileManagerController: REST controller for new agent file management endpoints
• AgentFileManagerPaths: Constants class for new agent-specific API paths

New Endpoints

All endpoints return FileDownloadDTO containing file metadata and downloadable resources:

1. GET /api/file-manager/get-content/general-agent-families-kb

   • Retrieves general agent family knowledge base files with content

2. GET /api/file-manager/get-content/specific-agent-family-kb

   • Retrieves KB files for a specific agent family with content
   • Query param: agentFamily

3. GET /api/file-manager/get-content/specific-agent-knowledge-documents

   • Retrieves knowledge documents for a specific agent with content
   • Query params: adminUid, agentFamily, agentUid

4. GET /api/file-manager/get-content/agent-message-attachments

   • Retrieves message attachments for an agent with content
   • Query params: adminUid, agentFamily, agentUid